How can I access files on a Pixelbook?

I have successfully installed the LibreOffice Linux app on my Pixelbook and was happy to see my old friend. However, I’m unable to access folders or files from both my Chrome clouds (including Dropbox) and external HD. I also seem to need an ID, password, and PIN. Is there a “LibreOffice on Chromebooks for Dummies” avaiable that could guide me with this?